System for compressing bandwidth



June 7, 1960 N. FREEDMAN ET AL SYSTEM FOR coMPREssING BANDWIDTH 2 Sheets-Sheet l Filed Aug. 24, 1954 N o n m7, 0 NRT Mw HN da. 7N HE NK wf Y B wi 3%@ NSG@ NR June 7, 1960 N. FREEDMAN ETAL 2,939,918

SYSTEM FOR coMPREssING BANDwInTH 2 Sheets-Sheet 2 Filed Aug. 24, 1954 A' x v5.3 @MNR INVENTORS. Nn ffm/v FQEEoMn/v Kin/wem I ma xm BY `Nathan Freedman,

.mediate frequency bandwidth receivers whose intermediate frequency bandwith is -dar beacon receiver.

2,939,938 Patented June 7, 1960 2,939,918 SYSTEM FOR COMPRESSING BTH Auburndale, and Kenneth T. Larkin, Wayland, Mass., assignors, by mesne assignments, to the Umted States of America as represented by the VSecretary of the Navy Filed Aug. 24, 1954, Ser. N0.`451,991 7 Claims. (Cl. MQW-15.55)

This .invention concerns a method and a system for compressing bandwidth, and more particularly, it con cerns a method and a system Vfor compressing the internecessary in wide-coverage wider than is necessary for fidelity reasons alone.

This invention is particularly adapted for use in a ra- The wide bandwidth of Va radar beacon receiver is made necessary by the fact that interrogating transmitters may scatter in frequency over a Vconsiderable region. This invention is adapted for use .with any receiver which is required to listen in slmultaneously on a wide band of .frequencies for possible transmissions, although the signals themselves only occupy a narrow spectrum bandwidth. There is no inherent frequencylimitation in this system; however, its

specific advantages are of most use in highfrequency,

very Wideband receivers.

An object of this invention is to compressing bandwidth.

yA further object is to provide a method for compressing intermediate frequency bandwidth in a wide-covera receiver to slightly more than one-half its original v ue.

vA further object isto provide a system for compressing bandwidth.

A further object is to provide a method for provide a systemfor compressing intermediate frequency bandwidth in a Wide-coveragereceiver to slightly more than one-half its original value.

therobjects and many -of the attendant advantages .of this invention will be readily appreciated as the same becomes better understood by referencerto the following 'detaileddescription when considered in .connection .with

the accompanying drawings wherein:

Fig. .1 is a schematic .wiring diagram of .apreferred embodiment of this invention with most components shown in block form,

Fig. 2 shows a graphicalnplot of the 'bandwidth input to 'the circutgof Fig. 1 and its division by the .branches of Fig. 1,

Fig. 3 is a graphical plot of the output frequencyband of'the'circuit of Fig. 1, and

Fig-4 isa block diagram of a invention. y

The embodiment of the invention shown in Fig. l

receiver including this 'includes a pair of isolated circuit branches 10 and '12. `The circuit branch includes a decoupling vacuum tube 16 at its input end anda decouplingy vacuum tube :18 atf-itsoutput end. The circuit'branch 12 includes a decoupling vacuum tube-22 at its input'end anda decou- @plingvacuum tube 24at its output end. The decoupling vacuuml tubes of the circuit branches llt) and 12 serve to decouple the circuit branches from each `otherto pre- `ventinteraction between the components in the circuit branches.V

YThe-input ends of circuit branches 10 .and 12-are adapted-Ito .be connected to a signal source 25; signal source 25 provides a narrow spectrum signal anywhere in a selected, very wide, acceptance frequency 2 band extending from frequency F1 2). The circuit branch 12 includes a filter 26. The filter 26 is characterized Iby a passband which includes the lower half of the bandwidth accepted from signal source 25 plus a small increment (A) ofthe uper half. The passband of filter v,26.extends from frequency F1 to to frequency F2 (Fig.

to frequency F2. Circuit branch 10 further includes a mixer stage I3,2. A local oscillator 34 is connected to one of the grids of the mixerstage 32. The output end of filter 28 is connected to another of the grids of mixer stage 32. The frequency generated yby the local oscillator 34 is equal ltothe sum of the frequencies defining the limits ofthe acceptance band offsignals derived from signal source25. The mixer stageA 32 heterodynes the signals yfrom 1filter 2S and from local oscillator 34. A third lter 42 is connected in the plate circuit of the mixer stage 32. VvThe filter 42 is characterized yby a passband which accepts the difference beat frequency from mixer stage 52. The b audpass of filter 42 extends `from frequency-E110 Fri-F2 V,Filter 2S operates with a sharp low frequency cutoff.

Filter 42 operates with a ksharp high frequency cutoff. The output end lof filter 42 is connected to the input end of decoupling vacuum tube 18. tubes is desirable at least at one end so that there can be no undesirable effects due to feedback around the loop comprising the .two circuit branches 10 and 12.

An amplifier 52 is connected to the output ends of circuit branches 10 and 12. The passband of amplifier 52 is only slightly more than one-half the `acceptance bandwidth of the branches 10 and 12 to signals from source 25. The outputs 4from circuit branches 10 and 12 `are combined and amplified by amplifier 52. The output of the amplifier is an amplifiedvsignal, which may be detected, just 'as-in a conventional system. lf desired, more complex filters may be substituted to afford the isolation above afforded by the isolation vacuum tubes 16, 1S, 22, 'and 24, making the latter unnecessary. In other words, more involved band-dividing filters may be designed which split up an acceptance band in two sectionsv while maintaining a constant input impedance. These are similar to the cross-,over networks used in high-and-low frequency loudspeaker installations. The use of these might eliminatethe necessity for the input pair Aof vacuum tubes. There is shown in Fig. 4 in Ablock-form',therelationship of this compression v.circuitto thefremainderof a receiver. The; compression circuit inaccordancewith this invention is connected between ra preamplifier 110,2, if needed, and ,an IF amplifier 104. The pre-amplifier 102 isconnected torantennaY 106. The IF amplifier is connected to'detector` and video or audio circuits 108.

The operation of the circuit isV best illustrated by `a numerical example. VAIAn-inspection of the quantities `in- -volved in thisuumcrical example shows 4that the mixing action in the circuit branch 10 converts the upper half Decoupling by of the circuit acceptance band to signals from source 25 into the lower half of the band from source 25. To illustrate, let it be assumed that the acceptance frequency band extends from 10 to 90 megacycles., Further, let it be assumed that A is 5 megacycles. v Therefore, the passband of lter 26 extends from l0 to 55 megacycles. The passband of iilter 2S extends from 55 to 90 megacycles. The local oscillator frequency is equal to +9O or 100 megacyclcs. The diiference generated in the mixer stage 32 is equal to`l00(55 to 90) or 10 to 45 megacycles. As a result, Athe l0 to 9G acceptance frequency band from source 25 is lit-ted 'into a band of 10 to 55 megacycles. If the frequency increment A were Ynot employed in the filter design, the acceptance band of signals from source-ZS'would be divided exactly in half. -The lower half of the band would go through circuit branch 12 and the upper half of the band would go into the mixer stage 32 in circuit branch 10. Under thisI condition of operation aV frequency Vat or close to theV center of the input bandwould arrive at the output of circuit branches V10 and 12 since filter characteristics cannot be Vmade infinitely sharp. The output of circuit branches 10 and 12 would then contain frequency components which are very close together, Frequency components which would be close'together would'beat in; the outputv detector to produce Yundesirable low frequency modulation Von the desired signal. Therefore, by staggering the branch aclator for generating a frequency which is equal to the sum of the frequencies defining the limits of the predetermined frequency band, a mixer stage connected to said second filter and to said local oscillator, and a third filter connected to the output of said mixer stage and characterized by a passband for passing only lower sideband frequencies from said mixer stage; the amount delta being no larger than is'necessary'for preventing harmful low frequency modulation of the desired signal. A 2, A bandwidth compression circuit for a predetermined frequency -acceptance band comprising a signal source of said band, an amplifier, parallel circuits connecting said source to said amplifier, each of said circuits including therein two decoupling stages one near the beginning and `one near the end thereof, one of said circuits between `its said stages having a filter passing only signals of frequencies in the'range of a lower part of at least half of said frequency Yband plus an increment equal `to a s mallf-raction of the entire frequency range of said acceptance band, Yand the Yother of said circuits'having Y sequentially inseries therein,.beginningV immediately after ceptance bands by the increment A the acceptance band center area is passed only by branch 12, thereby precluding undesirable low frequency modulation on the desired signal due to the beat effect in the output detector. The magnitude of A selected depends upon the excellence of the filters; it also depends upon the harmful effects of a Vlow frequency beat in using the receiver.

The spurious low-frequency modulation may be re-Y duced to whatever extent necessary byrmakring A large enough. How iarge'it needs'to be depends on the width of the signal spectrum, and the sharpness of the filter characteristics. 1

The output frequencyesponse will be iiat only if the gain through both circuit branches 10 and 12 are equal. This may conveniently be arranged by setting the gain of a tube in the circuit.V lf the gain through all filters is'unity,

it follows that the mixer conversion gain isunity for flat frequency response. Y

The bandwidth is compressed to make possible amplication over a wide band Vusing a reasonable number of tubes. Gain-bandwidth considerations would force use of a very large number of tubes ina conventional IF design of 80 megacycles bandwidth, for example. With half the bandwidth, the number of tubes is cut by much more Vthan half. Y

Y The system described may be cascaded for further bandwidth narrowing.Y However, too much cascading generally lowers receiver sensitivity.

Obviously many modifications and variations of the 'present invention are possible'in Vthe light of the above,Y teachings. It is therefore to be understood that within the scope of the appended claimsfthe 'invention may be practiced otherwise than as specically described.

Weclaim: l. A jbandwidth compressionecircuit for a predetermined frequency band comprising; ,ariirst circuit branch, a second circuit branch, means for connecting said circuit branches in parallel and for isolating said circuit branches `against interactionwithyone another; said first n circuit branch including a first filter characterizedrby a passband corresponding to the lower partof the predeter- ,y

Y mined vfrequency band, the passband differing from onehalf the predetermined frequency band byanY amount delta; said second circuit branch including Ya second filter characterized by a passband equal toV thev difference between the predetermined 'frequency band and thepassband of said first filter, -both of said filters manifesting sharp cutoff at the separation frequency, a local oscilits said` first decoupling stage, a filter passingA only signal frequencies in the rangelfrom' said lower part `of said band plus said increment tothe upper limit of said band,

Vthen a mixer tube, andthen a tilterpassing only signal Vfrequencies Vinthe range `of saidrlower partminusrsaid increment, and an Voscillator connectedto said.' mixer tube and heterodyning the signal' frequencies passing berange vof the lowest frequency inthe band toat least half -of said frequency acceptanceV bandplus anV increment equal Vto a small fraction Vof the entire frequency range of said acceptance band,.and Athe otherof said circuits having sequentially in series therein rst a filter passing only signal frequencies inthe rangefrom said half of said acceptanceband plus said increment to the upper limit of said band, and then a filter passing only signalV frequencies in the Yrange fromrthe lowest frequency in said acceptance .baud to said about half of said acceptance band minus said increment, and means connected to said other of said parallel circuits between said filters in that parallel circuit, for heterodynng the signal frequencies passing the first filter in that said other parallel circuit with a frequency equal to the sum of -about .the lowest and highest limits 'of said acceptance band to Aa range within the ,said lower part of said acceptance band.

5. A- bandwidth vcompression circuit for a, predeteri mined frequency acceptanceV band comprising ka signal sourcefof saidV band, an amplifienparallel circuits Vconnecting said source to vsaid amplifier, 4one of said circuits having a jiilter Apassingonlyv signals of frequencies Vin the range of a lower rpart of at least half of said frequency 'acceptance band plusl an increment equal to a small fraction ofY theentireffrequency range of said Yacceptance band, and theother of said vcircuits having sequentially in Yseries thereinl -firsta filter :passing onlyVv signal frequencies in the range from saidrlovverl part of'saidban'd Vplus* said"irrcrementto the upper limit of ','said band and Ythen aliilterpassing only signal lfrequencies in VVthe range of said lower part minus said increment, andan oscillator connected 'to said other of said parallel vcircuits' Vfor heterodyning the signal frequencies'passing the first filter in'that said other parallel circuit with a frequency'equal `'to the sum of the limit frequencies of Vsaidfacceptance bane," f

6. The circuit as set forth in claim 4, and means associated with the beginning and end of each of said parallel circuits for decoupling them.

7. The circuit yas set forth in claim 5, and means associated with each of said parallel circuits for decoupling 5 them.

6 References Cited in the le of this patent UNITED STATES PATENTS Di Toro Oct. 9, 1956 

